1. Field of the Invention
Embodiments in accordance with the present disclosure are directed to integrated circuits containing non-volatile memory cell arrays and particularly those arrays incorporating passive element memory cells.
2. Description of the Related Art
Materials having a detectable level of change in state, such as a resistance or phase change, are used to form various types of non-volatile semiconductor based memory devices. For example, simple antifuses are used for binary data storage in one time field-programmable (OTP) memory arrays by assigning a lower resistance initial physical state of a memory cell to a first logical state such as logical ‘0,’ and assigning a higher resistance physical state of the cell to a second logical state such as logical ‘1.’ Some materials can have their resistance switched back in the direction of their initial resistance. These types of materials can be used to form re-writable memory cells. Multiple levels of detectable resistance in materials can further be used to form multi-state devices which may or may not be re-writable.
With reference to FIG. 1, materials having a memory effect such as a detectable level of resistance are often used as a state change element 102 which is placed in series with a steering element 104 to form a memory cell 100. Diodes or other devices having a non-linear conduction current are typically used as the steering element. In many implementations, a set of word lines and bit lines are arranged in a substantially perpendicular configuration with a memory cell at the intersection of each word line and bit line. In FIG. 1, conductor 110 may form a word line and conductor 112 may form a bit line or vice-versa. Two-terminal memory cells can be constructed at the intersections with one terminal (e.g., terminal portion of the cell or separate layer of the cell) in contact with the conductor forming the respective word line and another terminal in contactor with the conductor forming the respective bit line. Such cells are sometimes referred to as passive element memory cells.
Two-terminal memory cells with resistive state change elements have been used in three-dimensional field programmable non-volatile memory arrays because of their more simple design when compared to other three-terminal memory devices such as flash EEPROM. Three-dimensional non-volatile memory arrays are attractive because of their potential to greatly increase the number of memory cells that can be fabricated in a given wafer area. In monolithic three-dimensional memories, multiple levels of memory cells can be fabricated above a single substrate, without intervening substrate layers.
One type of three-dimensional memory utilizes a rail-stack structure to form the memory cells. A rail stack is formed by creating successive layers of material which are etched together to form an aligned stack of layers. A memory cell may be formed at the intersection of two such rail stacks. The fabrication of rail-stack structures generally requires fewer mask layers and processing steps to implement an array than other memory structures. The unintentional programming of unselected memory cells is possible in rail-stack structures, particularly with respect to memory cells adjacent to those currently selected. Exemplary memory arrays utilizing rail stacks are described in U.S. Pat. No. 6,631,085 and U.S. Pat. No. 7,022,572.
Another type of three-dimensional memory includes pillars of layers formed at the intersection of upper and lower conductors. Pillar based memory arrays are characterized by the separation of the various structures forming each memory cell from similar structures forming adjacent memory cells. FIGS. 2A-2B are perspective and cross-sectional views, respectively, of a portion of a traditional monolithic three-dimensional memory array. Both the word line and bit line layers are shared between memory cells forming what is often referred to as a fully mirrored structure. A plurality of substantially parallel and coplanar conductors form a first set of bit lines 162 at a first memory level L0. Memory cells 152 at level L0 are formed between these bit lines and adjacent word lines 164. In the arrangement of FIGS. 2A-2B, word lines 164 are shared between memory layers L0 and L1 and thus, further connect to memory cells 170 at memory level L1. A third set of conductors form the bit lines 174 for these cells at level L1. These bit lines 174 are in turn shared between memory levels L1 and memory level L2, depicted in the cross-sectional view of FIG. 2B. Memory cells 178 are connected to bit lines 174 and word lines 176 to form the third memory level L2, memory cells 182 are connected to word lines 176 and bit lines 180 to form the fourth memory level L3, and memory cells 186 are connected to bit lines 180 and word lines 184 to form the fifth memory level L5.
FIG. 2C is a cross-sectional view showing the materials used in forming one type of suitable memory cell for the structure of FIGS. 2A-2B. Memory cell 152 is formed in a pillar between bit line conductor 162 and word line conductor 164. The memory cell includes a p-i-n type diode steering element 102 having a heavily doped n-type region 122, intrinsic region 124, and a heavily doped p-type region 126. Between doped p-type region 126 and conductor 110 is a state change element 104. As just described, an antifuse state change element or a re-writable material having two or more detectable levels of resistance may be used. Other types of diodes such as p-n junction diodes can also be used.
The formation of pillar structures typically requires precise alignment in forming the small feature sizes of the structures. Numerous lithographical processes may be needed to define the pillar structures forming the individual memory cells. Exemplary memory arrays including pillar-based memory cells are described in U.S. Pat. Nos. 5,835,396 and 6,034,882, each of which is incorporated by reference herein in its entirety.
There remains a need for improved three-dimensional pillar designs and corresponding fabrication processes for forming the same in non-volatile memory array technologies.